Thin-film light guides often consist of a transparent film of high refractive index deposited on a substrate having a lower refractive index. Such waveguides are commonly formed by diffusing titanium (Ti) into a lithium niobate (LiNbO.sub.3) substrate. The in-diffused Ti guide can either take a wide planar form over the substrate or be confined to a narrow ribbon, in which case it tends to assume somewhat the characteristics of an optical fiber.
It has been calculated that a substantial portion of the light from a laser beam coupled into the waveguide is actually transmitted into the substrate, either at the coupling-in juncture or by leakage from the waveguide. These modes normally undergo total internal reflections in the substrate until reaching the coupling-out juncture, such as at a detector array. These reflected modes in Ti-diffused LiNbO.sub.3 waveguide systems used in integrated optical spectrum analyzers present a serious noise problem.
It is proposed that absorbing layers formed on the backside of the substrate is a solution in suppressing substrate modes. These substrate modes can be attenuated by lossy interference stacks, referred to as dark-mirror coatings or black interference reflectors. But, in the past, such were designed to operate in air and absorb effectively at light incident normal thereto over a broad wavelength band in the visible range. Such a dark mirror coating for this purpose is illustrated schematically in drawing FIG. 3, to which later reference and explanation will be made. See U.S. Pat. Nos. 2,478,385; 3,761,160 and 3,858,965 for examples of anti-reflection coatings.